Hey y'all. New here since reddit can't seem to figure out what is going on. I am a college student who decided to give it a go and finally fix my car because my service light was driving me crazy. I got a scan and it was P0300 and P0171 plus loud hissing in the bay. Other issues were oil crud all around the block. So I had a lot of issues hit me at once. Here is what I did so far: all new LIM, valve, UIM, and thermostat gaskets. New EGR pipe. New plugs, wires are 2 years old. I fixed the P0171 code and got rid of all the vacuum leaks so I am glad that was done. But P0303 is still there and driving me nuts. Here is what I did to address that: coil pack and wire from the junkyard, new plug, injector is clicking normal compared to the others, and that is it. I took some data on an OBD 2 but I have no idea how to read/interpret it. Ive been talking to a GM guy that specializes in 3800 and he told me after all this work, it could be an 02 or crank sensor. I just dont understand how that is possible because I feel like that would ignite a P0300 instead of my P0303. I have done everything I could think of with the isolated cylinder 3 issue. Any help would be great and happy to clarify or send the OBD data if needed.

Edit: The reason there is no code on this OBD 2 is because I was at Oreillys and had my door open for 5 minutes running the codes and my car just died. Wouldn't start. It has been happening a lot if I leave the door open for more than 5 minutes it'll just die. So that is why there is no codes because I just replaced with a new battery. But I can ensure you, itll be P0303.








. Here is what I did to address that: coil pack and wire from the junkyard, new plug, injector is clicking normal compared to the others, and that is it. I took some data on an OBD 2 but I have no idea how to read/interpret it. Ive been talking to a GM guy that specializes in 3800 and he told me after all this work, it could be an 02 or crank sensor. I just dont understand how that is possible because I feel like that would ignite a P0300 instead of my P0303. I have done everything I could think of with the isolated cylinder 3 issue. Any help would be great and happy to clarify or send the OBD data if needed.

Concerning the code P0303, it seems that you are saying that if the code is cleared, it quickly comes back. Is this the situation? Also, along with the code do you experience the symptoms of a misfire at that time or is there no driveability issue, just a Check engine light with code?

Thanks for the reply and the question. I knew I was forgetting something. So yes, when I cleared the code it came back after about 30 minutes of driving. When I threw the new battery in last night, it came on after about 10 minutes. In the beginning, before I changed the battery, the computer was reading an avg of 15.8 MPG. When I changed the battery last night, the computer clocked in 19.6 MPG from the ride home. I haven't had the chance to drive it since but it did feel a lot better. To answer your second question, its a little difficult to answer because the car has always been like this. I would say that I notice the misfire at idle more then driving. Watching the onboard computer MPG gauge, at idle I can see it start to drop relatively quickly. Maybe from the misfire? Or most likely the computer hasnt had the chance to collect enough data.

If you feel that there is a problem with cylinder 3, there are a number of things that could be explored, even beyond what you have already done. Let’* talk about spark plugs. 3800 engines seem to love AC Delco spark plugs and hate other brands such as Champion. This seems strange but I have been down this road many times. Next, used plugs tell a story. If you pull the plug from cylinder 3 and another easy to get to cylinder, see how they compare in appearance. Take a picture of each if you like. This info could be valuable in troubleshooting the issue. There is more but let’* cover just a little a this time.

I forgot to ask the year, make, model of your Buick and how many miles?

Cylinder 1 on the left Cylinder 3 on the right

Cylinder 1 on the left Cylinder 3 on the right

Both are AcDelco

2003 Buick Lesabre 130k miles

It is good to see AC Delco plugs. I am sorry if I was not clear concerning the portion of the plug that is most important to us. The portion of the plug that enters the combustion chamber is what may provide a clue. So, the picture would be from an end view of the spark plug where the electrode is located. As close up as possible of the cylinder 3 plug plus another to compare to. If you decide to do this, I would then swap out the plugs when reinstalling. This would give the advantage of the additional test of swapping out another plug to see if any improvement is made.
Speaking of another area, the fuel system, when was the fuel filter last changed?

That appears to be an enormous amount of gap on the plugs and did you set the plug gap?

I understand. The problem is that I did already changed out the plugs. The first time was a set for all around. Then P0303 came up so I went to autozone to pick up another plug to rule out that issue. Still have it obviously. I even changed the wires around twice. Nothing there either. Injectors are good. Coil Pack good. ICM is not changed but I heard that is rare to go out and the connections looked fine one those. Regarding the fuel filter i am not sure, I havent changed it since owning the car. Could you possibly explain your reasoning behind the fuel filter? I understand that you are going through a checklist most likely, but I am having a really difficult time understanding why this can't be an isolated issue resulting with the systems for cylinder three (injector 3, wire 3, coil pack 3, plug 3, compression 3, etc). Wouldn't a fuel filter through out a P0300 because it would affect all the cylinders? That is also why I had a hard time understanding why the GM guy thought it could be the O2 or crank sensor after everything I had already done because again, wouldn't that affect all the systems?

Yes I completely get where you are coming from and I send the same thing. Bought these plugs from amazon for $20 or so. They came gapped at .28 or something insane like that. I gapped them at .60 so thats why they look a little funky. If I push the electrode back over anymore, it screws up the gap.